1. Field of the Invention
The present invention relates to improvements in semiconductor switches.
2. Description of the Prior Art
Semiconductor switches as shown in FIGS. 1(a) and 1(b) are well-known. FIG. 1(a) is a circuit diagram of a Darlington transistor switch, which comprises a main transistor 1, an auxiliary transistor 2 connected between the collector and the base of the main transistor 1, and a control means 3 for controlling the base current to the auxiliary transistor 2. FIG. 1(b) shows another switch circuit comprising a main transistor 1, and a gate-turn-off thyristor 4 connected between the collector and the base of the main transistor 1, wherein a forward pulse (or a continuous current) is supplied to the gate G of the thyristor 4 to turn it on, and a reverse bias pulse is supplied to the gate G to turn it off.
In the circuit shown in FIG. 1(a), it is necessary to continuously supply a sufficient amount of base current during the on period. Furthermore, a sharp rise in the collector voltage is inevitable in an overcurrent region, resulting in a low capacity of the circuit against a peak current. Whereas, in the circuit shown in FIG. 1(b), the voltage drop in the thyristor 4 does not acutely increase in an overcurrent region and hence the collector voltage in the main transistor 1 does not acutely rise, resulting in a large capacity of the circuit against a peak current. In this circuit, however, difficulties are involved in turning off the thyristor 4, which necessitates intricate gate structure for the thyristor. In other words, the voltage drop in the thyristor 4 is large and the collector voltage drop in the main transistor 1 increases accordingly.
A principal aim of the present invention is to reduce the turn-off duty for the thyristor 4 which drives the base of the main transistor 1.